1. Field of the Invention
The present invention relates to miniature combined optical elements formed of glass.
2. Description of the Related Art
Conventionally, optical elements formed of glass are press-formed by placing a heated and softened glass material in a molding die and then heating and pressing the material. In the case where such press-formed optical elements are used for optical systems for reading compact discs (CD), the optical elements have a numerical aperture (NA) of 0.45 so that a laser beam has a small beam spot. The numerical aperture (NA) indicates a limit of light condensation when the optical system has no aberration. Red laser light has been used for conventional CDs with a laser wavelength of 780 nm. Such laser light is focused by optical elements.
Optical elements molded by the conventional molding technique can handle a laser wavelength of 780 nm or more and an NA of about 0.45 or less for reading CDs. However, when there is a need to further focus light to have a smaller beam spot, it is difficult for the optical elements to respond to the need. The NA is a value indicating the size of an aperture pupil with a constant focal length. The larger NA, the more focused laser light, achieving high-density recording on discs Recently violet laser light (blue laser) has been used, which is shorter in wavelength than red laser light, and optical elements having an NA of 0.6 or larger are used to read records on discs. When such optical elements are molded by the conventional press forming technique, a convex portion serving as an optical functional surface has a curved surface with a small radius of curvature (surface has a large curvature), so that the sharply curved surface is formed and the optical elements have to be small in size as much as possible in view of the installation space of the optical elements, resulting in difficult molding. It is difficult to work dies for molding such optical elements, and transfer is poor on a boundary between a flange and the convex portion having the sharply curved surface. Another technique is known in which instead of a single optical element, two optical elements are mounted in a lens-barrel and a combination of the two optical elements enables a high numerical aperture for further condensing laser light. However, in this case, it is necessary to use a spacer ring to adjust an interval of the two optical elements in the optical axis direction, thereby increasing the number of components with poor accuracy. As an example of the two combined optical elements, the following configuration is available: as shown in FIG. 1, two optical elements 32 and 33 which are formed of glass and have convex optical functional surfaces 30 and 31 are combined with the optical functional surfaces 30 and 31 facing each other. In this case, the optical functional surface 31 of the optical element 33 is protruded more than a join surface 34, resulting in a larger amount of press on the optical functional surface 32 of the other optical element 30 (to increase NA), poor transfer, and low accuracy. When grinding is performed to reduce the thickness of the flange of the optical element 33, even on the opposite side from the optical functional surface 31, it is necessary to hold the optical functional surface 31 to perform grinding, so that the protruding optical functional surface 31 becomes an obstacle during the holding for grinding.